It is known to use synthetic polymer particles in silver halide photographic elements to improve physical characteristics. In particular, polymer particles from 0.5 .mu.m (500 nm) to 10 .mu.m have found wide use as matting agents in an element to increase the surface roughness so as to reduce self-adhering of the material, to reduce sticking of the material to manufacturing and processing devices, to improve the antistatic properties of the material, and to improve the vacuum adhesiveness of the material in contact exposure to prevent Newton's rings. Polymer particles smaller than 500 nm obtained by emulsion polymerization technique (polymer latex particles) have found wide use as replacements for gelatin. For example, it has been proposed to use polymer latex particles in both hydrophilic light sensitive layers and hydrophilic light insensitive layers to improve the element dimensional stability, to improve element drying characteristics during photographic processing, to improve layer adhesion and flexibility, to reduce pressure fog, to control dye and image stability, to carry photographically useful compounds such as dyes, couplers, accelerators, hardeners, etc., and to improve the scratch and abrasion resistance of the layer, in particular the surface protective layer.
Many techniques for synthesizing matting agents have been disclosed. Most commonly, a suspension polymerization technique is used. U.S. Pat. No. 5,492,960, for example, describes a microsuspension polymerization process to make polymer particles larger than 1000 nm. Polymer particles smaller than about 1000 nm, and especially smaller than about 500 nm, are typically made by emulsion polymerization. The resultant particle slurry is called latex. In emulsion polymerization, ethylenically unsaturated monomers are added to an aqueous phase which contains surfactant above the critical micelle concentration and a water-soluble initiator. The mechanism of the polymerization process has been the subject of much research and is generally agreed to include emulsification of monomer into a continuous aqueous phase to form monomer droplets having a size of about 1 to 10 .mu.m and diffusion of the monomer from the monomer droplets into surfactant micelles where the actual polymerization proceeds. Homogeneous nucleation will also occur for recipes with low surfactant concentration or monomers of relatively high water solubility, but polymerization in the monomer droplets is deemed insignificant. Thus, monomer droplets are formed to a size much larger than the resultant polymer particles and function solely as reservoirs holding the monomer until it diffuses into the growing micelles where the free radical polymerization takes place.
A conventional emulsion polymerization process prepares particles having a size and size distribution very sensitive to the type and amount of surfactant, initiator concentration, and decomposition kinetics. It does not allow a direct control of particle size and size distribution by control of monomer droplet size and distribution.
Polymer latex particles made by emulsion polymerization are typically electrostatically stabilized by using anionic surfactants. Some of the charges on the polymer particle surface may also come from the water soluble initiators used. Colloidal particles which are solely electrostatically stabilized are known to be destabilized by the presence of ions such as those in coating solutions. This is particularly so for coating solutions used to form photographic elements. It is very common to include in photographic elements various addenda, such as salts, sensitizing dyes, surfactants, thickeners, inorganic fillers, etc. The presence of these compounds in coating solutions significantly reduces the stability of polymer latex particles by reducing the electrostatic repulsion force from the interaction between electrical double layers or surface charges on the particles. Surfactants or sensitizing dyes may carry opposite charges to those on the polymer particle surface leading to latex particle flocculation through charge neutralization. This can have a significant impact on manufacturing processes such as filtering and delivering of the coating solutions. The efficiency of the coating process is therefore reduced.
In the manufacturing of photographic products, gelatin is widely used as a binder and, in solution, as a medium for the preparation of coating melts. Conventional lime-processed gelatin typically contains a significant amount of calcium ion concentration. Polymer latexes are greatly destabilized when added to coating solutions comprising gelatin, which results in manufacturing difficulties for making such coating compositions.
It is known to use sulfonic acid containing monomers in latex particles to improve the latex stability and compatibility with gelatin. However, the latexes are also known to cause coating solution viscosity increase, and degrade film physical properties such as ferrotyping resistance at high temperature and relative humidities. It is also known to prepare latex polymer particles at high percentage solids and large particle size by using a combination of ionic and nonionic surfactants. However, some nonionic surfactants are photographically active. Some ionic surfactants can significantly impact coating solution viscosity.
Recent patents have disclosed loaded latex dispersions, e.g. in which a photographically useful compound such as a coupler is loaded into the latex polymer particle. The usual procedure for preparing loaded latexes as described in US Pat. Nos. 4,203,716, 4,304,769, and 4,368,258 is to combine a solution of the photographically useful compounds in a water miscible organic solvent with the aqueous latex. The resulting mixture, which typically has about a 1:1 ratio of water to organic solvent, is diluted with water and organic solvent is removed by evaporation. Removing the water miscible solvent subsequent to loading apparently requires large scale processing equipment and lengthy processing times, which increases the expenses.
U.S. Pat. No. 5,536,628 describes a process for incorporating absorbing dyes into a pre-formed latex polymer particle. In the process a polymer latex of known solids is heated with stirring to 70 to 80 degree C. The absorbing dye is heated until it reaches its liquid state and is mixed with the polymer latex at high shear to generate an emulsion. The emulsion is then passed through a high energy homogenizer at least once to form an absorbing dye impregnated latex polymer dispersion.
The processes described above can result in incomplete loading which leaves, for example, residual dyes in the aqueous phase, which can then crystallize or form large oil droplets during storage generating coating spot defects.
The present invention provides photographic elements containing polymer particles smaller than 400 nm where the size of the polymer particles can be reproduced from run to run, where compatibility with gelatin is improved and where incorporation of photographically useful addenda is facilitated.